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Preimage Resistance

Preimage resistance is the property of a hash function that, given a hash value h, it is infeasible to find an input x with H(x)=h. One-wayness.

Purpose

Lets a hash function be safely used for password storage and commitment schemes — the input cannot be recovered from the hash.

Mechanism

For an n-bit hash, the best generic attack takes about 2^n brute-force trials, so a 256-bit hash is preimage-secure. Note: collision resistance ≠ preimage resistance; MD5's collisions are broken but its preimages remain hard.

Use cases

  • Password hashing (one-way storage)
  • Hash commitment schemes
  • HMAC security proofs
  • Blockchain PoW

Examples

  • Input 
    H(x)=target
    Output 
    2^n trials needed to find x

    One-wayness

Common errors

Confusing collision resistance with preimage resistance
They are independent properties; assess each separately

Related terms

Collision Resistance
Collision resistance is the property of a hash function that it is infeasible to find two distinct inputs producing the same hash output. The birthday attack dictates the required output length.
GLOSSARY
MD5
MD5 is a 128-bit cryptographic hash function designed by Ronald Rivest; proven insecure by collision attacks, it is now used only for non-security purposes such as checksums.
GLOSSARY
SHA-256
SHA-256 is the 256-bit-output member of the SHA-2 family, widely used in TLS, blockchains, and cryptographic applications, with no known practical attacks.
GLOSSARY

Related tools

Hash Calculator
Compute MD5, SHA-1, SHA-256, or SHA-512 hashes with hex or Base64 output. Free, private, and 100% client-side.
TOOL

References